df: do not treat rootfs specially

df: do not treat rootfs specially

Like any other pseudo file system, df should show rootfs only
when the -a option is specified, i.e. specifying -trootfs alone
is not sufficient.  As the rootfs entry is now elided by the
general deduplication in filter_mount_list (commit v8.20-103-gbb116d3),
all other references to rootfs can be removed again.

* src/df.c (show_rootfs): Remove global variable.
(ROOTFS): Remove constant.
(filter_mount_list): Remove case to handle rootfs specially.
(main): In the case for handling the -t option, remove setting
of the show_rootfs variable.
* tests/df/skip-rootfs.sh: Adapt the test case "df -t rootfs":
the rootfs file system must not be printed (because no -a).
* doc/coreutils.texi (df invocation): Correct the documentation
about eliding mount entries: it is not the first occurrence of
the the device which wins, but now rather the entry with the
shortest mount point name.  Also adapt the description about
eliding pseudo file system types like rootfs.
* NEWS (Changes in behavior): Adapt entry.